This invention relates to subcarrier fiber optic communication systems and radio communications systems; specifically to systems that utilize non-linear behavior to achieve harmonic upconversion of bandpass signals. This disclosure details a method for overcoming the generally inherent phase and amplitude distortion on harmonic upconverters by employing predistortion on the bandpass signal that is harmonically translated in the upconversion device.
The transmission of information signals cover radio or other bandpass channels is dependent on the process of modulation. This process requires that the information signal be translated to a suitable carrier frequency that allows the information to be transmitted and detected at the required destination without interference from or with other signals. The standard method for frequency conversion in radio systems is where the information signal and a suitable carrier frequency signal (in this case called a Local Oscillator or xe2x80x98LOxe2x80x99) are combined in a manner that causes the spectral content of the baseband information signal to be translated in the frequency domain from the origin to the range of the LO. An example is the case of cellular telephone transmissions, which use frequency modulation (FM transmission of a voice signal on a radio frequency of approximately 900 MHz. The voice signal has baseband spectral components that fit within a bandwidth of 200 to 3500 Hz whereas the LO is near 900 MHz. The voice or xe2x80x98basebandxe2x80x99 signal is converted to a time varying electronic signal that controls the frequency of the LO in a very narrow range such that the instantaneous frequency of the LO changes linearly with the level of the electronic voice signal. Due to the fact that FM requires more bandwidth than that occupied by the information signal the total bandwidth occupancy of this new carrier signal at the LO or carrier frequency is on the order of 30 kHzxe2x80x94more than the original baseband signal but still much less than that of the carrier. Thus the modulated carrier comprises a narrowband bandpass signal. The modulated carrier is then amplified and transmitted into the radio channel to be received by the called party in the cellular coverage region. At the destination the bandpass signal is translated back to a baseband signal where it is converted to an audible voice signal.
The modulation process may be sectioned into 2 basic functions: Converting the basic lowpass information signal to a bandpass signal and then translating this bandpass signal to a suitable frequency for transmission. These functions are often undertaken in a sequential manner in which the information signal is placed on an intermediate frequency (IF) and then, in a second or multiple operation, the IF is translated through further frequency translation operations to the desired carrier frequency. Often the second function is the most difficult process to achieve, especially if the required transmission frequency is extremely high.
An alternative solution to the problem of attaining extremely high carrier frequencies is found in harmonic upconversion. In this method a bandpass signal at a frequency much lower than the desired carrier frequency is produced in the standard IF mixing operation. The resulting bandpass IF signal is then passed through a non-linear device that produces an ensemble of harmonics of the input signal. The desired harmonic output from the device is captured with a bandpass filter and serves as the upconverted bandpass signal. The relationship between the harmonically upconverted carrier signal and the input carrier signal is determined by the specific harmonic number that was captured at the harmonic upconverter output, which is always an integer. For example, upconversion to a fifth harmonic means that for an input carrier signal at a frequency of fc, the output carrier is at a frequency of 5fc.
Harmonic frequency translation is carried out in two-port devices in which the transfer characteristic is inherently non-linear. A general expression for the output from such a deice for a continuous wave (CW) input (in this case cos(2xcfx80fct)) is given by                               R          ⁡                      (            t            )                          =                              ∑                          n              =                              -                ∞                                      ∞                    ⁢                      xe2x80x83                    ⁢                                    C              n                        ⁢                          exp              ⁡                              (                                  jn                  ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      f                    c                                    ⁢                  t                                )                                                                        (        1        )            
where Cn is a coefficient determined by the nature of the non-linearity and fc is the fundamental input frequency. This is based on the assumption that a periodic input causes a periodic output. Additionally implied is the fact that no sub-harmonics of the input fundamental frequency are of interest in the Fourier series of the output signal. Frequency translation is achieved by capturing one of the higher order output harmonics with a bandpass filter. The bandpass filter removes all but the harmonic component corresponding to the desired frequency at the network output, and so it could be said that the response of the network to the input CW subcarrier signal is one of pure frequency translation of the original signal. If in the case of a modulated input, the information characteristics can be preserved through the frequency translation process, a system benefit is accrued in certain situations.
Radio on Optical Non-Linear Upconverters
Fiber optic communication systems have typically been used in broadband configurations to trunk large amounts of baseband information over long distances. The larger bandwidth and relatively low loss characteristics of optical fiber have made it useful for the efficient transport of large information bandwidths at relatively low cost. Lately, fiber optic subsystems are finding more application in systems that were typically designed for electronic radio. CATV, Radar and some cellular and PCS subsystems now employ optical links that allow system operators to extend the coverage region or move more TV channels over wider distribution areas. Emerging Multipoint Communications Systems such as local multipoint distribution services (LMDS) will involve the delivery of broadband signals to residential sites via radio carriers in the 20 to 60 GHz range. The use of fiber is likely to increase greatly when these new services place enormous demands on existing service delivery systems and subscriber loop infrastructure. The latter systems, which are the focus of this document, primarily operate in a subcarrier mode where the signal that is modulated onto the optical carrier is itself a modulated carrier wave that upon detection (or optical demodulation) will once again be a modulated radio (or radio compatible) bandpass signal.
The harmonic upconversion potential for optical devices is well characterized and has been extensively studied. See
Afshin S. Daryoush, Peter R. Herczfeld, Zygmond Turski, Pradeep K. Wahi, Comparison of Indirect Optical Injection Locking Techniques of Multiple X-Band Oscillators, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 12, pp 1363-1369, December 1986.
Wake, D., Smith I. C., Walker N. G. Henning, I. D., Carver, R. D., xe2x80x9cVideo Transmission Over A 40 GHz Radio-Fibre Link,xe2x80x9d Electronics Letters, Vol. 28, No. 21, pp 2024-2025.
J. J. O""Reilly, P. M. Lane, R. Heidemann, R. Hofstetter, xe2x80x9cOptical Generation of Very Narrow Linewidth Millimeter Wave Signalsxe2x80x9d, Electronics Letters, vol. 28, no. 25, pp. 2309-2311, December, 1992.
J. J. O""Reilly, P. M. Lane, M. H. Capstick, H. M. Salgado, R. Heidemann, R. Hofstetter, H. Schmuck, xe2x80x9cRACE 2005: Microwave Duplex Optical Antenna Linkxe2x80x9d, IEE Proceedings-J, vol. 140, no. 6, pp. 385-391, December 1993.
O""Reilly, J. J.; Lane, P. M. Fibre-supported optical generation and delivery of 60 GHz signals, Electronics Letters, Vol 30, No. 16, pp 1329-1330, Aug. 4, 1994.
Tom Young, Jan Conradi, Wayne Tinga and Bob Davies. xe2x80x9cGeneration and Transmission of FM and xcfx80/4 DQPSK Signals at Microwave Frequencies Using Harmonic Generation and Optoelectronic Mixing in Mach Zehnder Modulatorsxe2x80x9d, Tenth International Confference on Integrated Optics and Optical Fiber Communicationxe2x80x94Technical Digest, vol. 4, pp. 72-73, June 1995.
Tom Young, Jan Conradi and Wayne Tinga, xe2x80x9cGeneration and Transmission Of FM and xcfx80/4 DQPSK Signals at Microwave Frequencies Using Harmonic Generation and Opto electronic Mixing in Mach-Zehnder Modulators.xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, vol. 44, no. 2, pp. 446-453, March 1996.
Bob Davies and Jan Conradi, Hybrid Modulator Structures for Subcarrier and Harmonic Subcarrier Optical Single Sideband, Phototonics Technology Letters, Vol. 10, No. 4, April 1998.
Bob Davies, Jan Conradi, Compatible Optical Single Sideband Modulator Structures for Wireless Applications, Wireless 97, July, 1998.
Robert J. Davies Jan Conradi David Dodds, xe2x80x9cHybrid single Sideband Optical Modulatorxe2x80x9d, Ser. No. 08/873,967, Filed Jun. 12, 1997.
Ali Motamedi, R. Vahldeick, xe2x80x9cGeneration of fourth harmonic microwave signals using Mach-Zehnder modulatorsxe2x80x9d, Optical Fiber Conference OFC-97 Technical Digest, pp. 354-355, February 1997.
Bob Davies and Jan Conradi, xe2x80x9cHybrid harmonic subcarrier optical single sideband with phase predistortionxe2x80x9d, Electronics letters, vol. 34, no. 17, pp. 1674-1675, August 1998.
Robert Olshansky, xe2x80x9cOptical Modulator For Cancellation of Second-Order Intermodulation Products In Lightwave Systemsxe2x80x9d, U.S. Pat. No. 5,239,401, August 1993.
Winnall, S. T.; Lindsay, A. C.; Knight, G. A., xe2x80x9cWide-band microwave photonic phase and frequency shifter,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, vol. 45, no. 6, June 1997.
Hoshida, T.; Tsuchiya, M. xe2x80x9cBroad-band millimeter-wave upconversion by nonlinear photodetection using a waveguide p-i-n photodiode,xe2x80x9d IEEE Photonics Technology Letters, vol. 10, no. 6, pp. 860-862, June 1998.
Braun, R. P.; Grosskopf, G.; Rohde, D.; Schmidt, F. xe2x80x9cMicrowave generation and transmission using optical heterodyning or optical upconversion,xe2x80x9d Proceedings of the 1996 International Topical Meeting on Microwave Photonics, pp. 53-56, Kyoto, Dec. 3-5, 1996.
Braun, R.-P.; Grosskopf, G.; Meschenmoser, R.; Rohde, D.; Schmidt, F.; Villino, G xe2x80x9cOptical harmonic upconversion for microwave generation in bidirectional broadband mobile communication system,xe2x80x9d Electronics Letters vol. 33 no 22 October 1997.
Chen; Chung Y., Newberg; Irwin L., xe2x80x9cFrequency multiplying electro-optic modulator, configuration and method,xe2x80x9d U.S. Pat. No. 5,040,865 Aug. 20, 1991.
Disclosures of prior art outline many types of non-linear optical link/harmonic upconverters. A clearer conceptualization of the details of signal predistortion is gained by observing a typical optical link configured for harmonic upconversion.
There is provided according to the invention a method and apparatus for harmonically upconverting a phase and/or amplitude modulated subcarrier signal to a higher frequency through a non-linear optical link is disclosed. The method requires that a bandpass signal, with a carrier frequency at a submultiple of the desired radio transmission frequency, be frequency translated in a non-linear two-port device. To avoid the inherent distortion inflicted on the information signal in harmonic upconversion, a predistortion technique is outlined. This consists of phase compression in the case of a phase modulated signal, amplitude scaling in the case of amplitude only and combined phase-amplitude predistortion for QAM type signals With predistortion on the subcarrier signal input to the non-linear two-port, the resulting upconverted signal may be transmitted in a standard form suitable for demodulation at the signal destination without special equipment.